1. Field of the Invention
The present invention relates to a linear motion rolling guide unit which is applied to sliding portions of machining tools, precision processing equipment and testing equipment and which consists of a track rail with raceway grooves formed longitudinally extending on both side walls thereof, a casing saddling the track rail and having raceway grooves, and a number of rolling elements that roll between the raceway grooves.
2. Description of the Prior Art
A conventional linear motion rolling guide unit is such that a relatively heavy equipment is mounted on a slider and the slider is moved forward and backward on track rails over a relatively long distance at high precision by the guide of a guide unit. A conventional linear motion rolling guide unit will be described referring to FIG. 12 and FIG. 13.
FIG. 12 is a perspective view of an example of the linear motion rolling guide unit. The linear motion rolling guide unit consists of two parallel track rails 2 secured to a bed 20, a plurality of sliders 1 saddling each of the track rails 2, and a slide table 4 secured to the sliders 1. Equipment placed on the slide table 4 is moved forward and backward in the directions indicated by arrow G. Each of the track rails 2 has raceway grooves 5 formed longitudinally extending on both side wall surfaces 21. The sliders 1 are mounted astride the track rails 2 so that they are slidable. Each slider 1 consists of a casing 3 which is slidable relative to the track rail 2 and has a raceway groove formed in position facing the raceway groove 5, a number of rolling elements like balls confined between the opposing raceway grooves to allow relative motion of the casing to the rail, and end caps 6 attached to the longitudinal ends of the casing 3, the longitudinal direction being the same as the sliding direction of the casing 3.
The sliders 1 are freely slidable on the track rails 2 because of the provision of the rolling elements that circulate along the raceway grooves 5 of the track rails 2. The rolling elements in a load region in each slider 1, traveling along the raceway groove 5 of the track rail 2, are led into a direction changing path described later which is formed in the end pap 6 and further moved into a return path formed parallel to the raceway groove in the upper part of the casing 3, so that the rolling elements circulate in an endless circulation path. By the rolling of the rolling elements under load between the raceway grooves of the sliders 1 and the raceway grooves 5 of the track rails 2, the sliders 1 can slide relative to the track rails 2.
In the guide unit shown in FIG. 12, the raceway grooves and the rolling elements engage with each other with no internal gap or in a preloaded condition because it is necessary to support radial and moment loads that acts through the sliders 1 in the vertical and lateral directions. Errors in parallelism, if any, of the two track rails 2 will result in an increase in the load during sliding motion, reduction in service life and variations in sliding resistance, thereby seriously affecting the precision and performance of the guide unit. Improving the precision of parallelism of the track rails 2 causes the number of assembly processes to increase. It is, however, very difficult to fix the track rails 2 in parallel with a high accuracy of less than several tens of micron over the entire length of several meters.
FIG. 13 is a sectional view showing the structure of a conventional linear motion rolling guide unit. The guide unit has a slide table 44 directly mounted astride the track rails 41, 42, with a large number of endlessly circulating bearings 46, 47 secured to the slide table 44. One of the track rails 41 is clamped by four bearings 46 from four directions to sustain the vertical and lateral loads while the other track rail 42 is clamped by two bearings 47 from above and under so that errors of parallelism between the track rails 41, 42 can be absorbed by the two bearings 47 slightly moving on the upper and lower raceway surfaces of the rail 42 in the directions indicated by arrow N. In such a structure, a number of endlessly circulating bearings 46, 47 each having a relatively large height are secured to the slide table 44. Then many lubricating oil passages 43 must be formed in the slide table 44, making the structure complex and large and therefore expensive. Not only does this make the unit unsuitable for applications to relatively small equipment but also makes it impossible to use the existing linear motion rolling guide units and track rails.
A linear motion guide unit similar to the above guide unit of FIG. 13 is disclosed in Japanese Patent Laid-Open No. 60938/1991. In the linear motion guide unit, one of the paired, parallel linear rails is made up of a vertical support section and a horizontal guide section. One side of a movable member guided along the two linear rails, is supported slidably on the linear rail in such a way that its lateral movement is restrained by the other linear rail. The other side of the movable member is supported slidably on the one linear rail in such a manner that it is restrained by the horizontal guide section of the linear rail from moving in vertical directions but is movable in horizontal directions. That is, when the parallelism of the paired linear rails cannot be ensured, the linear motion guide unit allows the second side of the movable member to be moved laterally relative to the first linear rail through a slider, thus making the assembly of the rails easy. To describe in more detail, the linear motion guide unit has a first linear rail and a second linear rail formed along a reference surface projectingly provided from the support bed, and the second linear rail consists of a vertical support section and a horizontal support section. One side of the movable member guided along these linear motion rails is slidably supported on the first linear rail through a first slider in such a way that its lateral motion is restricted by the first linear rail. The other side of the movable member is slidably supported on the horizontal guide section of the second linear rail through a second slider in such a way that it is restricted from moving in the vertical directions by the horizontal guide section of the second linear rail but is allowed to move in the horizontal directions.
A linear motion rolling guide unit that solves the problems of the conventional linear motion rolling guide unit is disclosed in Japanese Utility Model Laid-Open No. 23233/1992. This guide unit consists of track rails and sliders mounted astride the rails, the sliders having rolling elements and raceways for the rolling elements. Each slider consists of a lower member and an upper member. The lower member has the rolling elements and a raceway and is movably engaged with the track rail. The upper member is for fixing the table and is mounted on the lower member with a gap for allowing a limited motion in a direction perpendicular to the longitudinal direction of the track rail. The limited motion gap is formed either between the rolling elements-which are provided between the upper member and the lower member--and the front and rear ends of the raceway formed between the two members, or between side plates secured to the sides of the lower member for preventing the upper member from coming off and the side walls of the upper member.
Even when there is an error of several tens of microns in parallelism between the track rail of the reference and an accompanying track rail laid parallel to it, it is possible to maintain a high precision operation performance and a high rigidity at all times without increasing the load of the sliders-which are mounted astride the track rails and slide connected with a single slide table and without causing variations in the sliding resistance, by constructing the guide unit on the accompanying track rail side as mentioned above. This facilitates the work of fixing the accompanying track rail which is assembled parallel to the reference track rail. This makes it possible not only to form the guide unit relatively compact but maintain as high a rigidity of the saddle structure of the track rails and sliders as those of conventional units. Furthermore, this guide unit also permits the use of the existing products of both track rails sliders mounted on the reference track rail.
In conventional linear motion rolling guide units, a large number of rolling elements that rotate in two rows on a single track rail each are in two-point contact with the wall surface of a raceway--which is formed by a raceway groove in each longitudinally extending side wall of the track rail and by a raceway groove in the casing. As a whole the rolling elements are supported by four-point contact.
The linear motion rolling guide unit, however, has the following drawbacks. Since the rolling elements are in four-point contact with the raceway, the raceway grooves must be machined to a high accuracy so that each of the rolling elements is in two-point contact with the two wall surfaces or raceway surfaces of the raceway groove formed in the track rail and at the same time with the two wall surfaces or raceway surfaces of the raceway groove formed in the casing. Another disadvantage is that when the guide unit slides on the track rails with the casing applied with an imbalance load, the sliders cannot slide smoothly on the track rails.